The endobenthos of the Bay of Valdivia (Ecuador) was investigated in August-September 1999. At high tide, Ecuadorean beaches are flooded by seawater. This water contains very rich shrimp populations (mostly juveniles), which form the source of the important shrimp aquaculture in Ecuador. Biological characterization of these beaches was never done, leading to the importance of this work This thesis is part of a IUS-VLIR-ABOS project, being a cooperation between the University Ghent (Belgium) and ESPOL (Escuela Superior Politechnica del Litoral, Ecuador). The goal of this study was to investigate structural characteristics of the endobenthos on an Ecuadorean beach and to relate these to beach profiles and sediment composition. In doing so, zonational patterns would be detected to be used as a first step for sustainable management of these beaches. Six transects, scattered over the entire Bay of Valdivia were once sampled for macro- and meiobenthos. The six different sampling areas for every transect are situated in the intertidal area, including the dry beach. The depth of the samples in the sediment was 15 cm for macrobenthos and 10 cm for meiobenthos. On every sampling area three replicates were taken. All 108 macrobenthos replicates were worked out, while for meiobenthos only 36 replicates were processed. With the help of two dimensionless parameters; the dimensionless fall velocity and the relative tide range, the six transects can be characterized as 'intermediate' beaches of the type "Low Tide Bar Rip". The average density over the entire bay is 62 individuals per m². The macrofauna of the sampled beaches consists mainly of Mollusca with a total average density of 286 ind/m² for the entire bay. The most abundant species is the gastropod Olivella semistriata. Crustacea form a smaller part (total average density: 67 ind/m²). Ten species of this taxon were found , which makes it the most diverse. Important species are: Haustorius sp., Excirolana braziliensis and Emerita rathbunae. The density of the Polychaeta species is much smaller than those of the two other taxa (total average density: 13 ind/m²). Mollusca, Crustacea and Polychaeta were reported to be the three most important taxa on sandy beaches The highest total macrobenthos density was reported for the zone between 1.4 m and 2.4 m height (compared to Low Mean Water Springtide, LMWS). Below 1.4 m, total macrobenthic densities on the different beaches remain stable. There is no density increase towards of the subtidal area. The diversity however raises continuously from the higher to the lower beach. Generally, it was noted that the three northern beaches contain as much higher densities and diversities than the three southern beaches. Different multivariate analyses were carried out on the density data. TWINSPAN, as well as clusteranalysis and ordination led to comparable results. The spatial patterns of the macrobenthos are mainly determined by height and sediment characteristics, i.e. percentage mud and percentage very fine sand. Three communities can be distinguished; a High Beach Community, an Intermediate Beach Communitx and a Low Beach Communitx . The High Beach Community is dominantly situated on the dry beach and is characterized by Crustacea with the isopod Excirolana braziliensis as the most dominant species (average 143 ind/m²). Additionally, ghost crabs (Ocypode) appear in large numbers above the high tide swash limit. Besides Crustacea, different Insecta species are also present. Comparable with the Intermediate Beach Community, the lower beach is dominated by the gastropod Olivella semistriata (average 575 ind/m², resp. 129 ind/m²) On the intermediate beach Crustacea are also abundant. Here Haustorius sp. reaches its maximal average density, with namely 91 ind/m². Besides Mollusca, other taxa, such as Polychaeta, Crustacea, Nemertea and Echinodermata are found on the lower beach. Polychaeta have their maximum dist